Interface materials and methods of production and use thereof

ABSTRACT

An interface material comprising a resin mixture and at least one solder material is herein described. The resin material may comprise any suitable resin material, but it is preferred that the resin material be silicone-based comprising one or more compounds such as vinyl silicone, vinyl Q resin, hydride functional siloxane and platinum-vinylsiloxane. The solder material may comprise any suitable solder material, such as indium, silver, copper, aluminum and alloys thereof, silver coated copper, and silver coated aluminum, but it is preferred that the solder material comprise indium or indium-based compounds and/or alloys. The interface material, or polymer solder, has the capability of enhancing heat dissipation in high power semiconductor devices and maintains stable thermal performance. The interface material may be formulated by mixing the components together to produce a paste which may be applied by dispensing methods to any particular surface and cured at room temperature or elevated temperature. It can be also formulated as a highly compliant, cured, tacky elastomeric film or sheet for other interface applications where it can be preapplied, for example on heat sinks, or in any other interface situations.

[0001] This continuation-in-part application claims the benefit of U.S.utility application Ser. Nos. 09/398,989 filed on Mar. 9, 1999 and09/774,466 filed on Jan. 30, 2001 incorporated herein by reference intheir entirety.

BACKGROUND

[0002] Electronic components are used in ever increasing numbers ofconsumer and commercial electronic products. Examples of some of theseconsumer and commercial products are televisions, personal computers,internet servers, cell phones, pagers, palm-type organizers, portableradios, car stereos, or remote controls. As the demand for theseconsumer and commercial electronics increases, there is also a demandfor those same products to become smaller, more functional, and moreportable for consumers and businesses.

[0003] As a result of the size decrease in these products, thecomponents that comprise the products must also become smaller. Examplesof some of those components that need to be reduced in size or scaleddown are printed circuit or wiring boards, resistors, wiring, keyboards,touch pads, and chip packaging.

[0004] Components, therefore, are being broken down and investigated todetermine if there are better building materials and methods that willallow them to be scaled down to accommodate the demands for smallerelectronic components. In layered components, one goal appears to bedecreasing the number of the layers at the same time increasing thelayers routing density. This task can be difficult, however, given thatseveral of the layers and components of the layers should generally bepresent in order to operate the device.

[0005] Thus, there is a continuing need to: a) design and producelayered materials that meet customer specifications while minimizing thesize of the device and number of layers; and b) develop reliable methodsof producing desired layered materials and components comprising thoselayered materials.

SUMMARY OF THE INVENTION

[0006] In accordance with the invention there is provided an interfacematerial that comprises a resin mixture and at least one soldermaterial. The resin material may comprise any suitable resin material,but it is preferred that the resin material be silicone-based comprisingone or more compounds such as vinyl silicone, vinyl Q resin, hydridefunctional siloxane and platinum-vinylsiloxane. The solder material maycomprise any suitable solder material, such as indium, silver, copper,aluminum, tin, bismuth, gallium and alloys thereof, silver coatedcopper, and silver coated aluminum, but it is preferred that the soldermaterial comprise indium or indium-based compounds.

[0007] The interface material, or polymer solder, has the capability ofenhancing heat dissipation in high power semiconductor devices andmaintains stable thermal performance. It is not subject to interfacialdelamination or phase separation during thermal-mechanical stresses orfluctuating power cycling of the electronic devices in which it is used.

[0008] The interface material may be formulated by mixing the componentstogether to produce a paste which may be applied by dispensing methodsto any particular surface and cured at room temperature or elevatedtemperature. It can be also formulated as a highly compliant, cured,tacky elastomeric film or sheet for other interface applications whereit can be preapplied, for example on heat sinks, or in any otherinterface situations.

[0009] It may be also additionally useful to incorporate antioxidants toreduce oxidation of the polymer-based resins, wetability enhancingagents to promote wetting of surfaces, curing accelerators, such asthose accelerators that would allow curing at room temperature,viscosity reducing agents to enhance dispersability and crosslinkingaids. It is also sometimes desirable to include substantially sphericalparticles of filler to limit the compressibility of the interfacematerial in interface applications, i.e. to limit or control thethickness of the material and of the layer.

[0010] It has been also found that thermal conductivity of soldersystems, such as a combination of filler and the combined resin mixturediscussed above, can be especially improved by incorporating carbonmicro fibers, with other fillers, into the system.

DETAILED DESCRIPTION

[0011] An interface material can be produced that comprises a resinmixture and at least one solder material. The resin material maycomprise any suitable resin material, but it is preferred that the resinmaterial be silicone-based comprising one or more compounds such asvinyl silicone, vinyl Q resin, hydride functional siloxane andplatinum-vinylsiloxane. The solder material may comprise any suitablesolder material or metal, such as indium, silver, copper, aluminum, tin,bismuth, gallium and alloys thereof, silver coated copper, and silvercoated aluminum, but it is preferred that the solder material compriseindium or indium-based compounds.

[0012] As used herein, the term “metal” means those elements that are inthe d-block and f-block of the Periodic Chart of the Elements, alongwith those elements that have metal-like properties, such as silicon andgermanium. As used herein, the phrase “d-block” means those elementsthat have electrons filling the 3d, 4d, 5d, and 6d orbitals surroundingthe nucleus of the element. As used herein, the phrase “f-block” meansthose elements that have electrons filling the 4f and 5f orbitalssurrounding the nucleus of the element, including the lanthanides andthe actinides. Preferred metals include such as indium, silver, copper,aluminum, tin, bismuth, gallium and alloys thereof, silver coatedcopper, and silver coated aluminum. The term “metal” also includesalloys, metal/metal composites, metal ceramic composites, metal polymercomposites, as well as other metal composites. As used herein, the term“compound” means a substance with constant composition that can bebroken down into elements by chemical processes.

[0013] Interface materials, as described herein, have several advantagesdirectly related to use and component engineering, such as: a) theinterface material/polymer solder material can be used to fill verysmall gaps on the order of 2 millimeters or smaller, b) the interfacematerial/polymer solder material can efficiently dissipate heat in thosevery small gaps as well as larger gaps, unlike most conventional soldermaterials, and c) the interface material/polymer solder material can beeasily incorporated into micro components, components used forsatellites, and small electronic components.

[0014] Resin-containing interface materials and solder materials,especially those comprising silicone resins, that may also haveappropriate thermal fillers can exhibit a thermal capability of lessthan 0.5 cm²° C./w. Unlike thermal grease, thermal performance of thematerial will not degrade after thermal cycling or flow cycling in ICdevices because liquid silicone resins will cross link to form a softgel upon heat activation.

[0015] Interface materials and polymer solders comprising resins, suchas silicone resins, will not be “squeezed out” as thermal grease can bein use and will not display interfacial delamination during thermalcycling. The new material can be provided as a dispensable liquid pasteto be applied by dispensing methods and then cured as desired. It canalso be provided as a highly compliant, cured, and possiblycross-linkable elastomer film or sheet for pre-application on interfacesurfaces, such as heat sinks. Advantageously, fillers with a thermalconductivity of greater than about 2 and preferably at least about 4w/m° C. will be used. Optimally, it is desired to have a filler of notless than about 10 w/m° C. thermal conductivity. The interface materialenhances thermal dissipation of high power semiconductor devices. Thepaste may be formulated as a mixture of functional silicone resins andthermal fillers.

[0016] A vinyl Q resin is an activated cure specialty silicone rubberhaving the following base polymer structure:

[0017] Vinyl Q resins are also clear reinforcing additives for additioncure elastomers. Examples of vinyl Q resin dispersions that have atleast 20% Q-resin are VQM-135 (DMS-V41 Base), VQM-146 (DMS-V46 Base),and VQX-221 (50% in xylene Base).

[0018] As used herein, the term “compliant” encompasses the property ofa material that is yielding and formable at room temperature, as opposedto solid and unyielding at room temperature. As used herein, the term“crosslinkable” refers to those materials or compounds that are not yetcrosslinked.

[0019] As an example, a contemplated silicone resin mixture could beformed as follows: Component % by weight Note/Function Vinyl silicone 75(70-97 range) Vinyl terminated siloxane Vinyl Q Resin 20 (0-25 range)Reinforcing additive Hydride functional  5 (3-10 range) Crosslinkersiloxane Platinum-vinylsiloxane 20-200 ppm Catalyst

[0020] The resin mixture can be cured at either at room temperature orat elevated temperatures to form a compliant elastomer. The reaction isvia hydrosilylation (addition cure) of vinyl functional siloxanes byhydride functional siloxanes in the presence of a catalyst, such asplatinum complexes or nickel complexes. Preferred platinum catalysts areSIP6830.0, SIP6832.0, and platinum-vinylsiloxane.

[0021] Contemplated examples of vinyl silicone include vinyl terminatedpolydimethyl siloxanes that have a molecular weight of about 10000 to50000. Contemplated examples of hydride functional siloxane includemethylhydrosiloxane-dimethylsiloxane copolymers that have a molecularweight about 500 to 5000. Physical properties can be varied from a verysoft gel material at a very low crosslink density to a tough elastomernetwork of higher crosslink density.

[0022] Solder materials that are dispersed in the resin mixture arecontemplated to be any suitable solder material for the desiredapplication. Preferred solder materials ate indium tin (InSn) complexes,indium silver (InAg) complexes and alloys, indium-based compounds, tinsilver copper complexes (SnAgCu), tin bismuth complexes and alloys(SnBi), and aluminum-based compounds and alloys. Especially preferredsolder materials are those materials that comprise indium.

[0023] Thermal filler particles may also be dispersed in the resinmixture. If thermal filler particles are present in the resin mixture,then those filler particles should advantageously have a high thermalconductivity. Suitable filler materials include silver, copper,aluminum, and alloys thereof; boron nitride, aluminum spheres, aluminumnitride, silver coated copper, silver coated aluminum, carbon fibers,and carbon fibers coated with metals, metal alloys, conductive polymersor other composite materials. Combinations of boron nitride and silveror boron nitride and silver/copper also provide enhanced thermalconductivity. Boron nitride in amounts of at least 20 wt. %, aluminumspheres in amounts of at least 70 wt. %, and silver in amounts of atleast about 60 wt. % are particularly useful.

[0024] Of special efficacy is a filler comprising a particular form ofcarbon fiber referred to as “vapor grown carbon fiber” (VGCF) such as isavailable from Applied Sciences, Inc., Cedarville, Ohio. VGCF, or“carbon micro fibers”, are a highly graphized type by heat treatment(thermal conductivity=1900 w/m° C.). Addition of about 0.5 wt. % carbonmicro fibers provides significantly increased thermal conductivity. Suchfibers are available in varying lengths and diameters; namely, 1 mm totens of centimeters in length and from under 0.1 to over 100 μm indiameter. One useful form has a diameter of not greater than about 1 μmand a length of about 50 to 100 μm, and possesses a thermal conductivityof about two or three times greater than with other common carbon fibershaving diameters greater than 5 μm.

[0025] It is difficult to incorporate large amounts of VGCF in resinsystems such as the silicone resin mixture discussed above. When carbonmicro fibers, e.g. (about 1 μm, or less, are added to the resins they donot mix well because of the need to incorporate a large amount of fiberrelative to the amount of the resins for beneficial improvement ofthermal conductivity. However, we have found that relative large amountsof carbon micro fibers can be added to resin systems that haverelatively large amounts of other fillers. A greater amount of carbonmicro fibers can be added to the resin when added with other fibers thancan be added alone to the polymer, thus providing a greater benefit withrespect to improving thermal conductivity of the thermal interfacematerial. Desirably, if carbon micro fibers are present in the resinsystem, the ratio of carbon micro fibers to polymer is in the range of0.05 to 0.50 by weight.

[0026] It may also be advantageous to incorporate substantiallyspherical filler particles to maximize packing density. Additionally,substantially spherical shapes or the like will also provide somecontrol of the thickness during compaction. Dispersion of fillerparticles can be facilitated by the addition of functional organometallic coupling agents or wetting agents, such as organosilane,organotitanate, organozirconium, etc. The organo metallic couplingagents, especially organotitanate, may also be used to facilitatemelting of the solder material during the application process. Typicalparticle sizes useful for fillers in the resin material may be in therange of about 1-20 μm with a maximum of about 100 μm.

[0027] To illustrate the invention, a number of examples were preparedby mixing the components described in Examples A through J below. Theexamples shown include one or more of the optional additions, e.g.,wetability enhancer. The amounts of such additions may vary but,generally, they may be usefully present in the following approximateamounts (in wt. %): filler up to 95% of total (filler plus resins);wetability enhancer 0.1 to 5% (of total); and adhesion promoters 0.01 to1% (of total). It should be noted the addition at least about 0.5%carbon fiber significantly increases thermal conductivity. The examplesalso show various physico-chemical measurements for the contemplatedmixtures. Polymer Solder/Interface Material Examples Example A B C D E FG H I J Silicone Mixture 16 5 8 5 5 5 5 5 4 4 Organotitanate 4 3 0 3 3 33 3 3 3 InSn 92 92 82 InAg 63 In 63 SnAgCu 92 82 SnBi 83 68 Al 80 10 2929 10 10 25 Modulus (MPa) 25 15 25 15 20 23 25 30 20 25 Viscosity(poises) 1400 500 1200 450 1500 1600 500 750 650 1700 Thermal 0.3 0.150.4 0.14 0.14 0.12 0.16 0.17 0.18 0.10 Impedence (cm² ° C./w) Thermal2.5 5.1 2.0 5.5 5.8 6.2 5.2 5.0 5.0 6.0 Conductivity (W/m ° C.)

[0028] Components organotitanate, InSn, InAg, In, SnAgCu, SnBi, and Alare presented as weight percent or as wt. %.

[0029] Example A contains no solder material and is provided forreference purposes. Organotitanate is functioning not only as a wettingenhancer, but also as a fluxing agent to facilitate melting of thesolder material during the application process.

[0030] The solder compositions for these examples are as follows:InSn=52% In (by weight) and 48% Sn (by weight) with a melting point of118C; InAg=97% In (by weight) and 3% Ag (by weight) with a melting pointof 143C; In=100% Indium (by weight) with a melting point of 157C;SnAgCu=94.5% Tin (by weight), 3.5% Silver (by weight) and 2% Copper (byweight) with a melting point of 217C; SnBi=60% Tin (by weight) and 40%Bismuth (by weight) with a melting point of 170C. It should beappreciated that other compositions comprising different componentpercentages can be derived from the subject matter contained herein.

[0031] The processing temperatures are as follows: Examples A-E=150C for30 minutes; Examples F, J and I=200C for 30 seconds and 150C for 30minutes; Examples G and H=240C for 30 seconds and 150C for 30 minutes.

[0032] Thus, specific embodiments and applications of interfacematerials and polymer solder materials have been disclosed. It should beapparent, however, to those skilled in the art that many moremodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the spirit of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced.

We claim:
 1. An interface material for electronic devices comprising atleast resin material and at least one solder material.
 2. The interfacematerial of claim 1, wherein the interface material further comprises atleast one wetting enhancer.
 3. The interface material of claim 1 whereinthe at least one resin material comprises a silicone resin.
 4. Theinterface material of claim 3, wherein the silicone resin comprises avinyl terminated siloxane, a reinforcing additive, a crosslinker and acatalyst.
 5. The interface material of claim 4, wherein the vinylterminated siloxane is vinyl silicone.
 6. The interface material ofclaim 4, wherein the reinforcing additive is vinyl Q resin.
 7. Theinterface material of claim 4, wherein the crosslinker comprises ahydride functional siloxane.
 8. The interface material of claim 4,wherein the catalyst comprises a platinum complex.
 9. The interfacematerial of claim 8, wherein the platinum complex is aplatinum-vinylsiloxane compound.
 10. The interface material of claim 2,wherein the wetting enhancer comprises an organo-titanite compound. 11.The interface material of claim 1, wherein the at least one soldermaterial comprises an indium-based alloy or compound.
 12. The interfacematerial of claim 11, wherein the indium-based alloy or compoundcomprises InSn, InAg or In.
 13. The interface material of claim 1,wherein the at least one solder material comprises a tin-based alloy orcompound.
 14. The interface material of claim 13, wherein the tin-basedalloy or compound comprises SnAgCu or SnBi.
 15. The interface materialof claim 1, wherein the interface material comprises one of an aluminumor an aluminum-based alloy or compound.
 16. The interface material ofclaim 1, further comprising a filler material.
 17. The interfacematerial of claim 16, wherein the filler material comprises carbon microfibers.
 18. The interface material of claim 1, wherein the material hasa viscosity that exceeds 450 poises.
 19. The interface material of claim1, wherein the material has a thermal impedance of less than 0.3 cm²°C./w.
 20. A dispensable paste of an interface comprising at least oneresin material and at least solder material.
 21. The dispensable pasteof claim 20, wherein the paste further comprises at least one wettingenhancer.
 22. The dispensible paste of claim 20, wherein the paste isuseful as an interface material for electronic devices.
 23. A sheet orfilm of an interface comprising at least one silicone resin material andat least one solder material.
 24. The sheet or film of claim 23, whereinthe sheet or film further comprises at least one wetting enhancer. 25.The sheet or film of claim 23, wherein the sheet or film is useful as aninterface material for electronic devices.
 26. A method of making aninterface material comprising combining at least one silicone resinmaterial with at least one solder material.
 27. The method of claim 26,further comprising adding at least one wetting enhancer to the interfacematerial.
 28. The method of claim 26 further comprising formulating adispensable paste of the interface material.
 29. The method of claim 27,further comprising formulating a dispensable paste of the interfacematerial.
 30. The method of claim 26 further comprising molding saidinterface material as a sheet or film capable of being cut to size andapplied as an interface between components in an electronic device. 31.A method of increasing the thermal conductivity of the interfacematerial of claim 1 comprising incorporating therein carbon micro fibersand at least one thermally conductive filler.
 32. A method according toclaim 31 wherein said carbon micro fiber is present in an amount of atleast about 0.5 wt. %, or in a ratio of carbon micro fibers to polymerof at least 0.05.
 33. A method according to claim 31 further comprisingadditionally incorporating a filler comprising at least one of silver,copper, aluminum, and alloys thereof; boron nitride, aluminum nitride,silver coated copper, silver coated aluminum, and carbon fibers; andmixtures thereof.